What are the top 3 ways to remove conformal coating from a localised area of the PCB?
It is possible to remove small areas of coating for 99% of conformal coatings without damaging the circuit board below.
The three main techniques generally used are:
- Local chemical stripping of the conformal coating
- Mechanical Abrasion of the conformal coating
- Soldering through the conformal coating
Each process has its own issues.
Local Chemical Stripping of conformal coating for localised repair
The simplest process for local area rework around a device for instance is using a cotton bud soaked in stripping fluid or a device such as a conformal coating rework pen to remove the coating. The process involves rubbing gently across the area to be removed, which will dissolve the conformal coating.
If the coating is fresh, it tends to come off in a matter of seconds, whatever the type of coating applied, since the coating is not fully cured. However, if the coating is old, having been coated many years ago, or it is fully cured then it could take a little longer and patience is required. Polyurethanes with full chemical resistance naturally can be tricky to strip with chemicals!
The key to stripping with chemicals is
- Ensure that the chemical stripper is compatible with the circuit board being stripped. No use causing damage to the PCB while removing the coating.
- Ensure that any active ingredients from the stripping process are inert after processing or removed by cleaning after rework to avoid corrosion effects on the board.
- Correct stripping solution selection is critical for compatibility with the coating and this is why conformal coating manufacturers have several stripping products.
Mechanical Abrasion of conformal coating for localised repair
Several systems on the market now exist which use the concept of “shot blasting” / erosion of the conformal coating from the circuit board. However aggressive this sounds it can actually be a very successful method for localised removal.
The principle is simple. Fire particles of material at the surface of the coating and erode it away. Using “soft” particles of material instead of hard helps ensure the coating erodes before the circuit board in most cases.
The key to Mechanical Abrasion of conformal coating is
- Care has to be taken in the operation of the system to avoid damaging the circuit below. If the coating and the board parts are a similar hardness then this can be very difficult and a skilled process.
- Ensure the system is designed to minimise static since the machine generates very high static charges, which must be neutralised.
- Ensure the PCB is cleaned after processing to remove all the debris.
Soldering through the conformal coating for localised repair
Localised rework of components that are conformal coated can be achieved using a soldering Iron.
The coating volatises away with the application of heat, leaving the joint clear of coating. Occasionally, with certain coatings, the material may char slightly but this is generally a cosmetic issue and you need to discuss this with your quality department and the material supplier.
The Key to Soldering through the conformal coating is
- Ensure that no gases are released that are toxic
- That the finish achieved is cosmetically acceptable
- The heat used does not damage the circuit board
For further information on conformal coating removal and stripping contact SCH to discuss your needs.
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